1. Field of Invention
The present invention relates generally to the field of position determination using Global Positioning, and more specifically to using Global Positioning to correct seismic data in light of vertical movements of sources and receivers in marine environments.
2. Related Art
Marine seismic exploration investigates and maps the structure and character of subsurface geological formations underlying a body of water. One or more streamer cables containing seismic receivers is deployed into the water behind a vessel, and one or more sources may be towed by the same or different vessel. Less than perfect knowledge of the actual positions of the source at the time of firing and receivers at the time of arrival of reflected seismic waves may result in less than acceptable seismic data.
When doing marine seismic imaging of the subsurface strata one needs to establish the position and depth of the seismic source and the receivers. Conventionally they have been referenced to the sea surface, but this has the disadvantage that the actual surface varies up and down with time and it is thus at a different distance from the seabed at the different times of the seismic experiments. This invention offers methods to measure the vertical position without reference to the actual sea surface. This allows the seismic data to be corrected for the wind driven sea surface waves, which the source (to a first approximation) rides up and down on, as well as for tidal waves. It has been shown that this effect is important for time-lapse applications using modeling studies. Laws, R. et. al., Rough Seas and Time-lapse Seismic, Geophysical Prospecting, 50, 195-208 (2002).
The Global Positioning System (GPS), administered by the United States, is a satellite-based positioning system useful in marine seismic exploration, and seismic surveys may employ multiple GPS receivers at strategic points in a spread to determine the surface position of a vessel, or buoys tethered to streamers and sources. However, this still does not provide knowledge of the actual position of the receivers on the streamers and the sources, as they are not at the surface. Thus, while GPS has been used for surface positioning in marine seismic data acquisition, its use for accurately determining actual vertical position of sources and receivers is not known.
From the above it is evident that there is a need in the art for improvement in determining the vertical position of seismic sources and receivers in fluid media. This would in turn enhance precision of vertical corrections for observed reflected seismic signals.